1. Field of the Invention
The present invention relates to a method for manufacturing a thyristor and a thyristor, and more particularly to a method for manufacturing a thyristor in which a lifetime of a minority carrier is controlled to increase a frequency and reduce a loss, and a thyristor formed by the same method.
2. Description of the Background Art
In recent years, withstand voltages and currents of a thyristor and a diode have been increased. By taking the thyristor as an example, a device having a standard in which a withstand voltage is 10 kV and an ON-state current is several hundreds A during the operation of the thyristor has been developed. In the thyristor, generally, it is a goal of development to increase a switching speed of ON-OFF control action, that is, a so-called frequency characteristic. In the thyristor, a time for performing turn-off (a turn-off time) is shortened so that the frequency characteristic can be increased. If the turn-off time is shortened, the resistance of a thyristor element is increased. Consequently, an ON-state voltage is increased during operation. Thus, the thyristor has the trade-off relationship.
When the ON-state voltage is increased, an energy loss is increased so that a calorific value is increased. For this reason, an increase in the calorific value caused by the energy loss cannot be ignored in the thyristor having the high withstand voltage and current. If the ON-state voltage is reduced too much, the turn-off time is increased. Therefore, it is necessary to improve the trade-off relationship between the turn-off time and the ON-state voltage.
Currently, a method for controlling a lifetime of a minority carrier in a semiconductor has widely been employed in order to shorten the turn-off time. The conventional method for controlling a lifetime of a minority carrier is divided broadly into the following two ways.
As a first method, a heavy metal is diffused into a silicon semiconductor (a heavy metal diffusion method). In this method, a deep level generated by a combination of the heavy metal and silicon is utilized as a recombination center. More specifically, the heavy metal is applied (or deposited) on the surface of a silicon substrate, and is then introduced into the silicon substrate by thermal diffusion.
As a second method, ionizing radiation such as an electron beam, a gamma ray or a light ion beam, for example, proton is irradiated on a silicon semiconductor. A deep level generated due to irradiating composite defects which are caused by irradiating the ionizing radiation is utilized as the recombination center. By controlling the energy of the ionizing radiation and an amount of irradiation thereof, the control effects of a lifetime of a minority carrier are adjusted. The first and second methods will be described below in detail, and problems thereof will be described below.
A lifetime .tau. of a minority carrier obtained by the first method makes a great difference between devices and between wafers due to the amount of the deposited heavy metal, a variation in a coefficient of diffusion into the silicon, and the like. Consequently, it is hard to perform technological control. If the lifetime .tau. of the minority carrier is excessively reduced, a forward voltage drop (a so-called ON-state voltage V.sub.on) is increased so that the consumed power of a semiconductor device is increased to depart from a standard. Consequently, characteristic failures are caused so that yield is lowered. Furthermore, the heavy metal diffusion method has problems that it is difficult to anticipate characteristics in advance by using means such as a previous test and correction cannot be performed when an amount of thermal diffusion becomes excessive. In addition, if a silicon substrate has crystal defects, the heavy metal causes grain boundary diffusion along the crystal defects. Consequently, non-uniform distribution is promoted by segregation. For this reason, there is a possibility that device operation might become unstable and deteriorated.
In the second method, in the case where ionizing radiation having an energy E is irradiated on the silicon substrate, a difference between inverse numbers of the lifetime of the minority carrier obtained before and after irradiation is proportional to a dose wherein the lifetime of the minority carrier obtained before and after the irradiation is represented by .tau.0 and .tau.1 respectively and the dose of the ionizing radiation is represented by .phi.. A coefficient of proportion is given as a coefficient of damage k. More specifically, the following relationship is formed. ##EQU1##
The silicon semiconductor on which the ionizing radiation is irradiated is damaged. The damage is broadly divided into a damage on a surface protective film interface and a damage (crystal defect) on the inside of a single crystal (that is, the inside of the silicon substrate). The damage of the surface protective film has a great correlation with the dose .phi. and a small correlation with the energy. The damage on the inside of the single crystal has a great correlation with the dose .phi., and the distribution thereof is affected by the energy.
Japanese Patent Application Laid-Open Gazette No. 3-245569 has disclosed that the damage of a surface protective film is restrained and a lifetime of a minority carrier caused by crystal defects on the inside of a single crystal is properly shortened if an electron beam or a gamma ray having an energy of 6 MeV or more is irradiated with a lower dose .phi., so that a frequency can be increased and a loss can be lowered without a deterioration in a withstand voltage.
However, the crystal defects formed by using the electron beam or gamma ray are distributed over almost a whole area in a direction of a depth of the inside of a semiconductor device. For this reason, the crystal defects are also formed in a position which is not positively related to the electrical characteristics of the semiconductor device. Furthermore, it is difficult to improve the trade-off relationship between an ON-state voltage and a turn-off time.
In order to solve such problems, the following method for controlling a lifetime of a minority carrier has been proposed.
For example, a method for controlling a lifetime of a minority carrier using proton and helium ions has been proposed in Document 1 (Y. Shimizu, Proc. of ISPSD'90 pp.231-235 "Application of a Proton Irradiation Technique to High Voltage Thyristors"), Document 2 (W. Wondrak. Proc. of ISPSD'88 pp.147-152 "PROTON IMPLANTATION FOR SILICON POWER DEVICES"), Document 3 (T. Nakagawa, Proc. of ISPSD'95 pp.84-88 "A NEW HIGH POWER LOW LOSS GTO") and the like.
These have the following characteristics. When light ions such as proton are implanted into a semiconductor at a high energy, crystal defects which are locally generated in the vicinity of a range position are utilized as recombination centers of a minority carrier. By regulating an accelerating energy, the range position in a thyristor is controlled. By regulating the amount of ion irradiation, the lifetime of the minority carrier is controlled.
In the thyristor having a withstand voltage of 10 kV described above, however, a thickness of a silicon substrate has reached several millimeters and the proposal of the above-mentioned documents has been insufficient.
For example, FIG. 14 shows the relationship among a range position D.sub.ll, a spike voltage V.sub.DSP applied during turn-off and an energy loss E.sub.off, which has been presented by T. Nakagawa in the Document 2. FIG. 15 shows a partial structure of a GTO in the Document 2.
In the Document 2, a proton beam is irradiated from an N emitter layer (n.sub.E) side shown in FIG. 15, that is, a cathode plane (K plane) side, and the range position D.sub.ll is defined by a distance between a junction of a P base layer (p.sub.B) and an N base layer (n.sub.B) and the range position. As D.sub.ll is increased, that is, an accelerating energy is increased, the spike voltage V.sub.DSP applied during turn-off tends to be decreased and the energy loss E.sub.off tends to be increased. Consequently, it is apparent that the spike voltage V.sub.DSP and the energy loss E.sub.off have the trade-off relationship. Thus, another trade-off relationship appears in the technique proposed in the Document 2.
In the case where a thickness of the silicon substrate is increased, it is necessary to implant proton in a deeper position, that is, at a higher energy than in the prior art. FIGS. 16 and 17 show energy attenuation characteristics in the silicon substrate which are obtained by performing implantation at low and high energies. In FIGS. 16 and 17, an axis of abscissa indicates a depth of the silicon substrate, and an axis of ordinate indicates an amount of energy attenuation.
In the case where the implantation is performed at a low energy, most of the energy is attenuated in the vicinity of a range position R.sub.P as shown in FIG. 16. In the case where the implantation is performed at a high energy, energy attenuation is great in a surface position E.sub.S and an amount of the energy attenuation from the surface position E.sub.S to the range position R.sub.P cannot be ignored as shown in FIG. 17. It is supposed that such attenuation is caused by various mechanisms. As one of causes, when the proton has an energy of 4 to 5 MeV or more, it makes nuclear reaction with silicon atoms so that a reactive cross-sectional area is increased. The great energy attenuation from the surface position E.sub.S to the range position R.sub.P means that an amount of crystal defects generated from the surface position E.sub.S to the range position R.sub.P is also increased. Thus, there has been a fear that the locality of crystal defect distribution might collapse to lower the controllability of the lifetime of the minority carrier.
As a method for controlling a lifetime of a minority carrier using a light ion beam which is more advanced, the technique of Document 4 has been proposed (U.S. Pat. No. 4,056,408, Nov. 1, 1977 "REDUCING THE SWITCHING TIME OF SEMICONDUCTOR DEVICES BY NUCLEAR IRRADIATION"). According to the Document 4, separate implantation of an ion beam into forward and backward conducting portions of a semiconductor device (see FIG. 6 of the Document 4), separate implantation of an ion beam into the end and central active portions of the semiconductor device (see FIGS. 7 and 8 of the Document 4) and the like are performed to enhance the function of the semiconductor device. According to this technique, basically, light ions such as H or He are irradiated from one of plane sides of the semiconductor device so that an accelerating energy and an amount of irradiation can be optimized.
As described above, in the conventional method for manufacturing a semiconductor device, the lifetime of the minority carrier has often been controlled by implanting a proton or light ion beam having a high energy. However, the conventional method has problems that the energy is increased so that the spike voltage V.sub.DSP applied during turn-off and the energy loss E.sub.off have the trade-off relationship, and the locality of the crystal defect distribution collapses so that the controllability of the lifetime of the minority carrier is lowered.